This invention relates to pay telephone systems and in particular to coin telephone station sets wherein the customer may extend the call by depositing additional coins throughout the conversation period.
The prior art coin station arrangements generally fall into two broad categories. In the first of these categories the coin deposit interval is restricted to a designated period during the initiation of the call and at the end of an incremental charging period. If an initial coin deposit is required prior to called customer answer, this category operates in the prepay mode, whereas if deposit is required only after such answer, the system operates in the postpay mode. In either mode the calling customer should not deposit coins at any arbitrary time during the conversation since the coin operator cannot be informed that the customer has done so. The deposit-permissible indication, usually in the form of an announcement, that therefore must be provided at the end of the incremental charging period tends to be disruptive of the conversation and heretofore has been, at least to some customers, a source of unfortunate but, alas, irremediable annoyance.
The second of the aforementioned categories is one that has hitherto not seen significant usage in the United States. It employs a special type of coin station set which allows the calling customer to deposit coins throughout the call conversation. In prior art versions of this "escrow" type of coin station the rates for calls are calculated at the central office by translating the call signalling dial pulses sent by the calling customer. Rate analysis is conducted at the central office in accordance with various factors such as distance, time of day, etc. With the rate thus determined, the central office returns metering impulses, preferably uniformly spaced in time, to the station set to operate a coin collecting relay therein.
While the escrow coin station system allows for time independent coin deposits, it has the disadvantage that the metering pulses appearing on the loop during conversation manifest themselves to the conversing parties as annoying and disruptive audible clicks. To resolve this problem the prior art has resorted to transmission-improvement techniques that serve to attenuate this effect. For example, one arrangement provides a separate signalling path for the metering impulses by utilizing a phantom arrangement. Phantom signalling is described in Communication Engineering by W. L. Everitt, 1937, pages 315-317. Another prior art arrangement transmits the metering impulses at a high frequency within the voiceband frequency range thus rendering the clicks barely audible.
While these measures tend to eliminate the speech interruption problem, they require that a specialized and expensive transmission facility be dedicated, as well as the use of additional station set and central office equipment. In addition, neither prior art arrangement entirely eliminates speech interruption. Moreover, for the phantom arrangement to be effective in this regard, unless a near-identical match of the impedances at either end of the loop is maintained, isolation of the pulse-signal path will not be achieved. Similarly, the high frequency transmission arrangement would merely serve to attenuate clicks but not to eliminate them entirely. The possible alternate approach of moving the rate analysis function from the central office and relocating it at the station set also involves significant equipment additions which would entail even greater expense and would not allow for the reliable and timely updating of rate information.
It would therefore be advantageous to provide an arrangement and method for an escrow coin telephone circuit which retains the benefits of a centralized rate analysis arrangement based at the switching office while completely avoiding speech interruptions due to clicks that are generated when metering impulses are transmitted during the call.